A principle problem with fuels of a relatively low calorific value, e.g., 25 MJ/kg, or less is the lower flame speed that can adversely affect the completion of combustion, particularly for uneven fuel/air mixtures, thus affecting the local fuel/air ratio in the combustor. This problem is particularly pronounced in the case of liquid fuels, where the fuel/air mixtures may have large fuel particle (droplet) sizes, which increase the time required to vaporize and burn the particles.
The achievement of low levels of oxides of nitrogen in combustors is closely related to flame temperature and its variation through the early parts of the reaction zone. Flame temperature is a function of the effective fuel-air ratio in the reaction zone which depends on the applied fuel-air ratio and the degree of mixing achieved before the flame front. These factors are obviously influenced by the local application of fuel and associated air and particularly the effectiveness of mixing.
The use of film cooling in these low flame temperature combustors generates high levels of carbon monoxide emissions and eventually creates sediments. External impingement cooling of the flame tube (liner) can curtail such problems. Moreover, the requirement for stoichiometric combustion requires the air flow to the reaction zone be a small portion of the total air flow, and a large portion of the total air flow be available for the dilution zone. Hence there is a considerable advantage in controlling these flows to optimize the combustion efficiency and minimize the emissions.
Improvements are possible in the configuration of can combustors and in the control of air and air/fuel mixture flows in the can combustors using liquid fuel with a low calorific value, which flows affect the completeness of the burning, and thus the level of emissions and the thermal efficiency of the combustor. Such improvements are set forth hereinafter.